The Siemens 5SJ3120-7 is a 1-pole miniature circuit breaker from the SENTRON 5SJ3 series, rated 20 A at 240 V AC with a C tripping characteristic. It is designed for branch-circuit protection in final distribution boards, sized for a 70 mm installation depth and accepting any mounting position.
Rated 20 A at 240 V AC, this breaker protects a single-phase branch circuit up to that continuous current. The C-curve (5–10× In) provides a middle ground between the fast B-curve and the slower D-curve — it holds through the startup surge of a small motor or a bank of fluorescent ballasts but trips quickly on a hard short. Overvoltage category 3 and pollution degree 2 confirm suitability for fixed-installation distribution boards in a typical indoor environment. IP20 with connected conductors means the breaker is finger-safe only when wired; it requires enclosure protection against dust and moisture.
The 1-pole width occupies a single 18 mm module on the rail. Rated for 50 Hz supply frequency; the AC/DC voltage type means the internal arc-extinction design handles both AC and DC fault clearing, though the 240 V rating is specified for AC use.